A traditional radar technique to detect the moving speed of a target is utilization of the Doppler effect. This technique detects the moving speed of a target on the basis of the difference (hereinafter referred to as “Doppler frequency shift”) between the frequency of a radio wave transmitted (emitted) to the moving target and the frequency of a radio wave reflected off the target (hereinafter referred to as “reflected wave”).
The Doppler effect-based technique however is not capable of detecting the moving speed of the target when a radio wave is emitted in a direction orthogonal to the direction in which the target is moving (hereinafter “orthogonal direction”), because no Doppler frequency shift occurs.
Patent Literature (hereinafter, referred to as “PTL” 1) (hereinafter, referred to as “related art”) discloses a technique that detects the moving speed of a target with emission of a radio wave in the orthogonal direction, for example.
FIGS. 1A and 1B are diagrams schematically illustrating the related art. As shown in FIGS. 1A and 1B, the related art uses a radar having an array antenna disposed parallel to the moving direction of target 23. Target 23 is moving at a constant speed from left to right in the drawings. Each antenna element (Akl, Akm . . . ) constituting the array antenna emits radio waves in the orthogonal direction to a lateral surface of target 23 and also detect reflected waves from the target.
In FIG. 1A, dotted lines 24 represent the direction in which reflected waves propagate from target 23 at time t=t1. The term “dlm” indicates the distance (interval) between antenna elements. The reflected wave from target 23 positioned at distance R and angle θ1 at time t=t1 is received by each antenna element. At the time of reception, a radio wave channel path difference of dlm sin θ1 occurs between two given antenna elements, e.g., Akl and Akm. Consequently, reflected waves having a phase difference corresponding to the channel path difference are detected between the two antenna elements.
Similarly, assuming that the angle is θ2 at time t=t2 (t2>t1) in FIG. 1B, reflected waves having a phase difference corresponding to the channel path difference of dlm sin θ2 are detected at Akl and Akm.
In the related art, the moving speed of target 23 can be then determined by calculating the temporal differentiation of the phase difference detected between the two antenna elements.
In determination of the moving speed of target 23, the related art assumes that the position at which the radio wave transmitted from the radar is reflected at target 23 (hereinafter referred to as “reflection position”) is the same at times t1 and t2. In other words, the related art assumes that the radio wave is always reflected at the same position on the moving target.